As is well known, a phased array antenna has an array of identical radiators (waveguides, horns, slots, dipoles etc.) with electronic means for altering the phase of power fed to each of them. This allows the shape and direction of the radiation pattern to be altered without mechanical movement and with sufficient rapidity to be made on a pulse-to-pulse basis. Not surprisingly, the proper operation of a phased array antenna requires periodic monitoring for faults in the system, with the consequent need for calibration of misaligned components or the replacement of defective components. For such monitoring, the two most important performance parameters of the antenna are; 1) the radio frequency (RF) amplitude; and 2) the phase of each-signal path from each antenna radiator to the receiver. Furthermore, with accurate amplitude and phase information for each radiator element, other antenna performance factors, such as gain, monopulse null depth and sidelobe pattern can be determined.
Various attempts have been made in the past to provide some system for monitoring a phased array antenna. One known method incorporates a control loop for each array element. Unfortunately, these loops are complicated, bulky and relatively expensive. Another known method for monitoring phases array antennas uses coupler injected signals through the feed. It happens, however, that the accuracy of such a device can be questionable. In still another example, U.S. Pat. No. 4,468,669 for an invention entitled "Self Contained Test Device" discloses twin-lead transmission lines for signal injection but uses a phase toggling technique for fault isolation which was intended for passive array antennas. The present invention recognizes there is a need for an apparatus to monitor phased array antennas which is effective and reliable for use with both active and passive antennas.
In light of the above, it is an object of the present invention to provide an antenna calibration system which will maintain low sidelobes under operational conditions, e.g. while airborne. Another object of the present invention is to provide an antenna calibration system which is capable of performance monitoring, antenna calibration, fault isolation and fault correction for either an active or a passive phased array antenna. Still another object of the present invention is to provide an antenna calibration system which can be relatively easily incorporated into existing antenna systems. Yet another object of the present invention is to provide an antenna calibration system which is simple to use, relatively easy to manufacture and implement, and comparatively cost effective.